Nonaqueous secondary batteries have now enjoyed wide use as a power source of high voltage and high energy density with the recent spread of portable electronic equipment, such as notebook computers and handy video cameras. Nonaqueous secondary batteries have also found practical application to electric cars and hybrid cars which utilize electric power as part of driving power for considerations of environmental conservation.
However, nonaqueous secondary batteries suffer from a reduction in capacity and an increase in internal resistance in low temperature or as a result of repetition of charges and discharges. For this the reliability as a stable power source has been questioned.
Various additives have hitherto been proposed to improve the stability or electrical characteristics of nonaqueous secondary batteries. For example, JP-A-10-326611 makes a proposal to use tetramethyl silicate as an organic solvent for an electrolyte. The proposal brings about improved cycle characteristics but results in reduction of capacity. JP-A-10-55822 proposes an electrolytic solution which is made flame-retardant by using a silane compound, such as octyltriethoxysilane, as a flame-retardant organic solvent. The electrolytic solution proposed, while improving flame retardance, is not sufficiently satisfactory in electrical characteristics. U.S. Pat. No. 6,235,431 discloses an electrolytic solution containing an organosilicon compound having an Si—N bond, which prevents generation of a halogenic acid due to water penetration thereby preventing a battery from deterioration to some extent but is not sufficiently satisfactory in terms of electrical characteristics stability against charge and discharge cycles.
Further, JP-A-8-171936 teaches addition of an inorganic tin compound to a nonaqueous electrolytic solution of a lithium secondary battery, alleging that tin ions improve electrical characteristics. JP-A-10-69915 discloses addition of a stannic ester or a germanic ester to a nonaqueous electrolytic solution. These tin compounds or germanium compounds cannot be seen as sufficiently satisfactory in low-temperature battery characteristics.